Date: Tue, 03 Dec 2002 14:54:48 -0500
From: Wayland Hiott
To: Ralph Kenyon
Subject: Colloids...what ARE they?
Dear Ralph,
I am searching for a definitive answer about what colloids are. I am a high school physical science teacher. I am getting conflicting info from different sources, even from college texts.
Is a colloid homogeneous or heterogeneous or something in between?
I thought it was hetero (supported by my college chem text) due to particles not being uniform in size and being 1-200nm in size. My high school text says homo. A film we use in science says homo, and uses milk as an example. It does seem funny that homogenized milk would be heterogeneous!
Please help me get this straight!
Wayland Hiott
Gilbert High School

Date: Tue, 03 Dec 2002 22:59:21 -0500
Wayland Hiott
Ralph Kenyon
Subject: Re: Colloids...what ARE they?

Hi Wayland,

I assume, of course, that you have already read, and that is why you wrote me.

A very simple answer can be found at
You might find the entry "colloidal system" in the Van Nostrand's Scientific Encylopedia interesting.
You'll find more than you ever wanted to know at

I'm certainly not an expert. I would tend to look at the "problem" (What is a colloid?) from the point of view of what is being communicated. It's worth looking at the details of the context in which the term is used in order to disambiguate the intent of the communicator.

Colloidal sized particles are of a "uniform size" only in relation to the exponential scale from atoms to planets. But they also vary with regard to each other by as much as 200 to 1. Whether the size can be labeled by the abstraction "uniform" depends upon the scale you choose to view the particles from. Present the details and the abstract label becomes less meaningful or useful.

Similarly with the substance. It is the composition of the particles that are often more homogenous than not. The butterfat in milk tends to rise to the surface as cream. But when the same butterfat is broken by mechanical processes into small enough (colloidal sized) particles, it remains in suspension. I daresay that you might be able to separate the milk fat out using a centrifuge.

A "colloidal system" consists of one or more substances finely divided and dispersed within another substance. The result must exhibit certain properties, and different terms are applied depending upon what the final product is like. Some are called gels,
some are called emulsions, etc. Jello and mayonnaise are examples respectively of a solid dispersed within a liquid and a liquid dispersed within another liquid.

My inclination is not to ask what a colloid "is", but in what circumstances do we (have we) use(d) the term (and how has that usage changed over time).

Back in the twenties, before the double-helix was discovered, it was surmised that "colloids" were the primary basis of life. We now know that much of life's process involve mega-molecules in the size range which came to be associated with colloids of the original "Brownian motion" size - stuff that would stay suspended in water, but life's chemistry and physics involves so much more, as the modern anti-body lock-and-key metaphor illustrates. The "colloidal" size of these life molecules is now almost only a foot-note.

So the simple answer, for those who only want a short verbal definition to answer test questions, a colloid is a mixture of one or more substances finely divided and dispersed within another substance that presents a relatively uniform or homogenous appearance to the naked eye, and which will not separate out into its individual components when left alone for a relatively extended period of time. Examples can be any combination of the different states of matter (and I'm guessing at some of these) from whipped cream (gas in liquid), mayonnaise (liquid in liquid), Jello (solid in liquid), cement (solid in solid), fog (liquid in gas), etc. The substances in question are generally homogenous in themselves, and, to the naked eye, so is the combination. But the combination is not a chemical compound of the respective substances that make up the colloid.

If I go back in time and draw on my memory, the explanation was that a colloid is unlike a chemical compound - which is homogenous in substance. It is a heterogeneous mixture of two (or more) different substances, each of which is itself homogenous in nature. But the combination is so finely divide as to give the appearance of homogeneity to the observer.

Try having your kids research and answer the question by using the internet. Be cautioned that many of the explanations on the internet are from the homeopathic (and possibly fraudulent) "nutrition" supplement community hawking "colloidal mineral supplements" alleged to cure almost anything.

Best of luck, and don't hesitate to contact me in the future.

Ralph E. Kenyon, Jr.

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